1. Field of the Invention
This invention relates to a heater construction particularly suitable for use in a hot isostatic pressing apparatus.
2. Description of the Prior Art
The use of graphite is rapidly spreading in the manufacture of heaters for hot isostatic pressing apparatuses (hereinafter referred to simply as "HIP apparatus" for brevity) which treat a workpiece or workpieces in a high temperature and pressure atmosphere by application of pressure and heat, because of the advantages accruing from (1) the excellent heat resistance of graphite heaters so as to realize the feasibility of operations in a higher temperature range with a longer service life and with less deterioration than the conventional metal heaters, and (2) far lower cost as compared with the metal heaters.
To cope with the recent trend toward larger HIP apparatuses which require a heater of larger dimensions in both vertical and radial directions, there have come into use the so-called stacked type heater constructions which incorporate a number of heating element units in stacked form instead of a heating element of a unitary or single structure.
This is because large unitary type heaters are difficult to produce and need to be divided into a plurality of heating zones of different capacities in order to prevent temperature variations between different positions in the vertical or longitudinal direction of the furnace chamber in a high pressure container.
The conventional graphite heaters generally incorporate a grid-like meandering heating element which is formed by cutting longitudinal slits alternately from the upper and lower ends of a cylindrical structure as shown in FIG. 7(a), or by connecting round rods and flat strips into a zig-zag form as shown in FIG. 7(b). In the fabrication of a stacked type heater, it is the general practice to stack a number of such cylindrical heating unit structures through an annular electric insulator of a unitary or composite structure.
Such heater construction is suitable for a small HIP apparatus but not for a large HIP apparatus in view of the mechanical instability of the heating element 3' of graphite which is required to serve as the base structure of the heater, coupled with the economical problem that the cost of the apparatus is increased due to the necessity of providing a greater number of electric insulators 7. The application to a large industrial HIP apparatus is further limited by a problem concerning maintenance and service that the heater has to be disasembled almost in its entirety at the time of replacement of the heating elements.